Rf sputtering of insulator materials

ABSTRACT

METHOD AND APPARATUS FOR SPUTTERING ELECTRICALLY NONCONDUCTIVE MATERIAL WHEREIN POTENTIALS OF OPPOSITE POLARITIES ARE APPLIED TO A TARGET OF NONOCONDUCTIVE MATERIAL WHICH POLARITIES ARE ALTERNATED AT (RF) RADIO FREQUENCIES. THE SPUTTERING DISCHARGE MAY ALSO BE SUPPORTED BY RF POTENTIALS.

3 Sheets-Sheet 1.

+ INLET B. a. MECKEL ET L RF SPUTTERING 0F INSULATOR MATERIALS July 20,1971 Filed Sept. 19, 1966 OSCI LLATOR 52 Ill 9 E INVENTORS BENJAMIN B.MECKEL BERND H. RICHELMANN ATTORNEY July 20, 1971 MECKEL ETAL 3,594,295

RF SPUTTERING OF INSULATOR MATERIALS Filed Sept. 19; 1966 2 Sheets-Sheet2 INVENTORS BENJAMIN B. M ECKEL ATTORNEY United States Patent Oifice3,594,295 RF SPUTTERING OF INSULATOR MATERIALS Benjamin B. Meckel, LaMesa, and Bernd H. Richelmann, San Diego, Calif., assignors to PhysicsTechnology Laboratories, Inc., La Mesa, (Calif.

Filed Sept. 19, 1066, Ser. No. 580,404 Int. Cl. C23c 15/00 US. Cl.204-192 11 Claims ABSTRACT OF THE DKSCLOSURE Method and apparatus forsputtering electrically nonconductive material wherein potentials ofopposite polarities are applied to a target of nonconductive materialwhich polarities are alternated at (RF) radio frequencies. Thesputtering discharge may also be supported by RF potentials.

In the physical process of sputtering, atoms are ejected from a surfaceunder positive ion bombardment. The positive ions, being formed byelectrons colliding with gas molecules, bombard the cathode or targetand atoms of the target material are ejected. The atoms are ejected bythe direct momentum transfer of energy from the positive ion to theejected atom resulting in high velocity atoms produced by the kineticenergy released through the ion bombardment. The target material must bekept at a potential that is negative relative to the positive ions todraw the positive charge ions for bombardment. Where the target materialis a conductor; then the positive potential accumulated on the surfacebecause of the positive ions impacting against the target is drained offby the negative direct current power source. However, where the targetmaterial is a non-conductor, then the positive ions depositing on thesurface to create sputtering will accumulate sufliciently to eventuallycreate a positive charge that repels or inhibits further sputtering.

In conventional sputtering devices the plasma is produced by auxiliaryalternating or static magnetic or electrical fields. These fields mustbe closely related to and coordinated with the entire sputtering processand the target or cathode potential. The ejected atoms travel through asubstantially direct path to impact on surrounding structure, walls andfittings. Some of the ejected atoms return to the target itself afterhaving collided with gas molecules, however a sufficient portion of theejected atoms are deposited on a selected surface. In such conventionalsputtering processes, the sputtered atoms may not be pure becausecontaminating atoms from surrounding walls and fixtures or fromcontaminating gases can be mixed with the ejected atoms and thus thedeposited mate rial. For applications that require a pure film, such asin space electronics, it is necessary to preform the process in a vacuumand to reduce sputtering of surrounding walls and fixtures as much aspossible. However, the sputtering of surrounding walls and fittings isnot entirely prevented is not entirely prevented in such sputteringprocesses and adversely affects the thin film produced on the selectedsurface.

It is therefore an object or our invention to provide a novel andimproved method and apparatus for producing thin films.

It is another object of our invention to provide a novel and improvedmethod and apparatus for producing thin films by sputtering.

It is another object of our invention to provide a novel and improvedmethod and apparatus for producing thin films by sputtering in a plasmaproduced by radio frequency induction that is independent of thesputtering process itself.

It is another object of our invention to provide a novel 3,594,295Patented July 20, 1971 and improved method and apparatus for producingthin films by sputtering in a high density plasma in which the plasma isproduced by a high frequency magnetic field without the need for anyauxiliary alternating of static magnetic or electrical fields.

It is another object of our invention to provide a novel and improvedmethod and apparatus for producing thin films in which power is suppliedto the target by means of a single stage power oscillator means, thefrequency of which adjusts itself to various loading conditions andobviates the need for separate tuning adjustments.

It is another object of our invention to provide a novel and improvedmethod and apparatus for producing thin films which apparatus can usehighly reactive as well as inert (noble) gases without harm to theelectrodes.

It is another object of our invention to provide a novel and improvedmethod and apparatus for producing thin films that uses a separateinduction coil for the creation of the plasma and which coil isinsulated and shielded from the plasma.

It is another object of our invention to provide a novel and improvedmethod and apparatus for producing thin films that is simple andinexpensive to construct and is easy It is another object of ourinvention to provide a novel and improved method and apparatus forproducing thin films that is simple and inexpesive to construct and iseasy to operate.

The present invention comprises a vacuum chamber that is evacuated by apump. A gas supply means supplies gas to the chamber at a low butconstant pressure. A shielded and cooled induction coil is positioned inthe chamber and is supplied with high frequency current. The inductioncoil generates a high frequency magnetic field in its central volumethat produces an accompanying high frequency electric field forgenerating plasma. The shielding prevents unwanted radiation ofelectromagnetic energy. High frequency power for the induction coil issupplied by a single stage oscillator that tunes itself automatically tomatch all loading conditions and obviates the need for complex tuningadjustments.

The target to be sputtered is supported by a target holder that includesa pair of electrodes. If the target material is a conductor, then directcurrent that is negative to the potential of the plasma and surroundingstructures is applied to the target through the electrodes. If thetarget is a nonconductor, then high frequency alternating current isapplied to the two electrodes in such a manner that the instantaneouspotentials of the two electrodes are degrees out of phase. Thus separateportions of the nonconductor will be sputtered at separate intervals,that is, when the particular backing electrode is negative.

Ion bombardment during the interval of sputtering deposits a positivecharge on the surface of the non-conducting target. During this intervalthe other electrode is positive and the surface of the non-conductingtarget adjacent said other electrode attracts electrons out of theplasma. This effectively neutralizes the positive charge acquired duringthe previous cycle. Thus during such sputtering of non-conductors, oneof the target holder electrodes is always positive, drawing electronsout of the plasma to the surface of the non-conducting target, while theother electrode is negative by an equal but opposite amount, drawingions out of the same plasma to the non-conducting target surface. Thisinsures that the next potential of the plasma with respect tosurrounding fixtures remains zero, thereby avoiding the inhibiting ofsputtering of the target and reducing unwanted sputtering of adjacentstructures.

Other features and advantages of the present invention will become moreapparent from the following detailed description taken in conjunctionwith the accompanying drawings which illustrates and clarifies thepreferred embodiment of this apparatus, and in which:

FIG. 1 is a representative view of our invention for producing thinfilms by sputtering.

FIG. 2 is a cross section of the insulated and shielded high frequencyinduction coil assembly that produces the high density plasma.

FIG. 3 is a sectional view of the induction coil assembly taken fromFIG. 2 alo g line 33.

FIG. 4 is a sectional view of the electrode target holder assembly andshielded electrical supply line structure.

FIG. 5 is a sectional view of the electrode target holder assembly takenfrom FIG. 4 along line 55.

Referring now to the drawing wherein like reference characters designatecorresponding parts throughout FIGS. 1 through 5; there is shown in FIG.1 a thin film production apparatus including a thin film sputtering unit12. The sputtering unit 12 comprises a vacuum chamber or enclosedcontainer 22, such as a glass Pyrex bell jar, that is supported bysupport means 26. Positioned between support means 26 and container 22are section rings 28 and 30 that have flanges and seal means 32 forproviding a vacuum tight access through which the gas supply means 16and the cooling fluid supply means 18 and 20 pass. A vacuum pump means14 that comprises a diffusion pump 34 in combination with a roughingpump 36 is driven by a motor 38. The vacuum pump means 14 continuouslyremoves impurities, such as contamination and condensation particles andgases, from container 22 during operation of the thin film productionapparatus A gas, stored in a container 40, is continuously and slowlyreleased into the bell jar 22 through a throttling valve 42. Thisthrottling valve 42 is selectively adjusted so that a low but constantgas pressure is maintained in the bell jar 22 (for example, about 0.0005millimeter of mercury absolute). An induction coil 44 is located withinthe bell jar 22 and is supplied with high frequency current throughlines 46 and 48. Cooling fluid for cooling the conductors in line 18,such as water or compressed air, is carried in tubular insulatedconductors 46 and 48. A shielding structure 56 entirely encloses theconductors 46 and 48 and prevents unwanted radiation of electromagneticenergy in the enclosed container 22.

Induction coil 44, when energized with high frequency current, generatesa high frequency magnetic field in the volume in and near its hollowcenter that produces an accompanying high freqeuncy electric field. Thiselectric field ionizes the low pressure gas in the volume to produceplasma. The high frequency power for induction coil 44 is supplied bythe high frequency generator 52, which comprises a single stage poweroscillator that tunes automatically to match all loading conditions andthereby eliminates the need for complex tuning adjustments. The coil 44is enclosed in the ring shaped ceramic shielding member 54 in container22. Line 18 encloses the length of the conductors 46 and 48 that connectthe generator 52 with the coil 44. Line 18 also shields conductors 46and 48 to the junction box 58 that is connected to the ceramic members.A ceramic sealing disc 60 is provided at the junction box 58 for vacuumtight sealing of the conduit conductors 46 and 48 that passtherethrough.

The target 62 that is sputtered (see FIGS. 4 and 5), is supported by atarget holder means 63. The target 62 is in the plasma 50 even thoughthe target is spaced above coil 44. The target 62 is positioned parallelto and along the axis of the opening through the center of coil 44.Particles dislodged by sputtering from the target material 62 willtravel through the opening of coil 44 toward the substrate 66. In orderto form a desired pattern on the substrate 66, a mask 68 is positionedbetween target 62 and the substrate 66 so that the particles will travelthrough the openings of the mask 68 and impact and adhere to thesubstrate 66 forming a thin film pattern on its surface. The substrateand mask 66 and 68 are supported by a support means 70 that is securedin the container 22 above the diffusion pump 34.

The target holding means 63 is provided with holding devices or clips 72and a pair of half circular shaped electrodes 74 and 76. The clips 72hold the target 62 securely against the electrodes 74 and 76 and theelectrodes 74 and 76 are connected to an electrical power source bytubular insulated conductors 78 and 80 which also provide partialsupport for the electrodes. A cylinderical housing 20 shields theelectrical conductors 78 and 80 and prevents unwanted radiation of theelectromagnetic energy carried by the conductors. A switching means 82functions to selectively supply direct current from source 84 oralternating current from source 86 to conductors 78 and 80. Thus theelectrodes 74 and 76 are either supplied alternating current or directcurrent, depending upon the position of the switch 82.

The conductor 78 is shaped into an induction coil 92 after itselectrical contact with electrode 74. The tubular coil 92 completes thecooling fluid circuit while presenting a high impedance to highfrequency currents. A shroud 94 shields the clips 72 and the other sideof the electrodes 74 and 76 from the plasma 50 and prevents unwantedsputtering of walls and fixtures. The half circular shaped electrodes 74and 76 are separated by a small distance across the diameter of the halfcircle. For guidance of the sputtered atoms and confinement of theplasma, a shielding means 96 and 97 is secured between the targetholding means 64, the coil 44 and the support 70.

OPERATION In operation, the thin film sputtering unit 12 of the thinfilm production apparatus 10 receives a continuous supply of gas duringoperation. The bell jar 22 is evacuated by the vacuum pumping means 14and the gas that enters the jar 22 at a predetermined rate may be anygas suitable for creating ions in a plasma, for example, argon or thelike. The gas in passing through the jar 22 absorbs impurities such asmoisture, unwanted gases, contamination or molecular particles or thelike from the inside of the jar 22 and its associated structures. Thecontinuous admission of new gas and the continuous removal of old gascreates a pure, low pressure gaseous condition within the jar 22. Thediffusion pump 34 may be any conventional type, such as an oil ormercury diffusion pump system. The low pressure gas admitted into theevacuated jar 22 quickly diffuses to all parts of the jar 22 and isionized by the high frequency magnetic field produced by induction coil44.

The high frequency electrical conductors 46 and 48 and 7-8 and 80 carrycurrent to the jar 22 and are provided with pressurized fluid means,such as air or water through the hollow conductors, for cooling. Asshown in FIG. 1 a water supply inlet is connected through switch 82, andcirculates cooling water through the electrical conductors to a combinedoutlet.

While any type of suitable material can be sputtered in the present thinfilm production apparatus 10, such as conductive and nonconductivematerials, the mode of operation for sputtering conductive ornon-conductive materials is different. Assuming that the target materialis a conductor, as for example, copper, gold, silver, etc., and theinside gaseous pressure environment of the container 22 is at itsdesired value; switch 82 is set to apply a direct current to theelectrodes 74 and 76. The direct current has a constant negativepotential with respect to the plasma and the surrounding structures. Thepositive ions which are formed in the plasma are attracted to the nownegative electrodes 74 and 76 and bombard the target material 62 causingsputtering. The sputtered particles or atoms are ejected from the target62 as described hereinbefore, and travel through the opening in theinduction coil 44 towards the substrate 66. A mask 68, having apredesigned pattern screens the substrate 66 and forms a thin film witha predetermined pattern on the substrate 66. The negative potentialsource removes the positive charges deposited on the target material.

When the target material 62 is of a non-conducting material, as forinstance ceramic or the like, a high frequency current is then appliedto the electrodes 74 and 76 by the setting of the switch 82 in theproper high frequency position. The electrodes 74 and 76 are noWsupplied with instantaneous opposite potentials that are 180 degrees outof phase. This phase shifting is caused by the balanced output ofoscillator 86 and aided by the impedance of tubular coil 92 to theapplied high frequency current. Thus at any instance, one-half of thematerial 62 is negative and is thus sputtered while the other half ofthe non-conducting material is positive and repels bombarment of thepositive ions. Ion bombardment during this interval of sputteringdeposits a positive charge on the negative one-half surface of thenonconducting target 62 that is sputtered. In order that this positivecharge does not subsequently repel and inhibit further sputtering, theopposite electrode that was previously sputtered is positive andattracts electrons sputtered from the other half of the target. Thiseffectively neutralizes the positive charge acquired previously. Thus,during the sputtering of non-conductors, one of the electrodes is alwayspositive and draws electrons out of the plasma 50 to the surface of thenon-conductor material of the target, while the other electrode has anegative potential that draws ions out of the same plasma and causes thenon-conducting target to be sputtered. This action insures that thepotential of the plasma with respect to surrounding fixtures andstructures remains zero, and thus reduces the unwanted sputtering ofadjacent structures and/or fixtures.

It should be understood to anyone familiar with the art that the vacuumpumping means, the gas supply means and its associated components asshown herein can be replaced by any other vacuum or pumping meansproducing an equal result.

We claim:

1. In a sputtering apparatus, including a vacuum chamber,

means for admitting an ionizable gas into said chamber,

means for effecting an electrical sputtering discharge within saidchamber,

target holding means for supporting an insulator material in saidsputtering discharge to be sputtered,

wherein said insulator material is positioned so that a surface thereofcomes into contact with said sputtering discharge,

means for applying RF potentials to said target to cause said insulatormaterial to sputter;

the improvement wherein said means for applying a RF potential includesmeans for simultaneously impressing a first polarity current on a firstelectrode adjacent one portion of said surface and an opposite polaritycurrent on a second electrode adjacent another portion of said surfaceand alternating said polarities at RF.

2. In a sputtering apparatus as claimed in claim 1 wherein said meansfor applying a RF potential includes electrode means that include saidfirst and second electrodes for conducting alternating RF current,

said electrode means includes means for reversing the phase of thealternating RF current in the other electrode relative to the phase ofthe alternating RF current in the other electrode to cause each of saidelectrodes and space charges adjacent said respective surfaces of saidinsulator material to simultaneously have opposite polarities.

3. In a sputtering apparatus as claimed in claim 2 including,

said first and second electrodes are electrically connected by aninduction coil.

4. In a sputtering apparatus as claimed in claim 3 in which,

said insulator material has flat surface portions on its side oppositethe surface presented to said sputtering discharge,

and said first electrode contacts a first conductor plate that ispositioned immediately adjacent said opposite flat surface in an areacorresponding to said one portion of said surface and said secondelectrode contacts a second conductor plate that is positionedimmediately adjacent said opposite flat surface in an area correspondingto said another portion of said surface.

5. In a sputtering apparatus as claimed in claim 4 in which,

said conductor plates abut against said fiat surfaces,

said induction coil comprises a coil that is connected between and inseries with said first and second electrodes,

and said induction coil forms an inductive impedance between saidelectrodes of suflicient magnitude to substantially reverse the phase ofthe alternating RF current from one electrode to the other electrode.

6. In a sputtering apparatus as claimed in claim 4 including,

a housing positioned within said container that com pletely enclosessaid electrodes and conductor plates from said sputtering discharge.

7. In a sputtering apparatus as claimed in claim 1 including,

RF induction means for ionizing said gas into said sputtering discharge,

a first RF current source for supplying high frequency current to saidmeans for applying a RF potential,

and a second RF current source for supplying high frequency current tosaid RF induction means.

8. A method of sputtering in a vacuum chamber com prising the steps of,

admitting an ionizable gas into said chamber,

effecting a sputtering discharge in said chamber,

positioning an insulator material so as to contact said sputteringdischarge to sputter the surface of said insulator material,

simultaneously applying a first polarity current through an electrodeadjacent one portion of the surface of the insulator material and asecond polarity current through a second electrode adjacent anotherportion of the surface of the insulator material,

and alternating said polarities at RF.

9. A method of sputtering as claimed in claim 8 in which,

the opposite potential polarities are provided by RF alternating currentthat is phase shifted between the portions of the nonconductivematerial.

10. A method of sputtering as claimed in claim 8 in which saidsputtering discharge is produced by applying RF alternating currentexcitation to the interior of the container.

11. A method of sputtering as claimed in claim 8 including the steps of,

providing a continuous controlled flow of a given gas to said evacuatedcontainer during sputtering,

and continuously evacuating said given gas from said container duringsputtering.

References Cited UNITED STATES PATENTS 3,291,715 12/ 1966 Anderson204-298 3,309,302 3/1967 Heil 204192 3,369,912 2/ 1968 Davidse et al204-192 ROBERT K. MIHALEK, Primary Examiner US. Cl. X.R. 204-298

